1. Field of the Invention
The present invention relates in general to valve mechanisms for internal combustion engines, and more particularly to valve mechanisms for automotive internal combustion engines of a type which has a plurality of intake or exhaust valves for each cylinder. More specifically, the present invention is concerned with valve mechanisms of a type which can control the valve timing and the valve lift in accordance with the engine condition.
2. Description of the Prior Art
As is known, for improving fuel consumption and operability at lower speed and lower load condition of the engine and obtaining a sufficient output at higher speed and higher load condition of the same, various valve mechanisms of the above-mentioned type have been proposed and put into practical use. One of such valve mechanisms is shown in Japanese Patent First Provisional Publication 60-26109.
In order to clarify the task of the present invention, the valve mechanism of the publication will be described with reference to FIG. 10 of the accompanying drawings. The valve mechanism shown is incorporated with an intake valve 3 of the engine.
In the drawing, denoted by numeral 1 is a cylinder head of an internal combustion engine. The cylinder head 1 has at its upper deck a cam shaft 2 on which a valve driving cam 2a is provided. Positioned near the cam shaft 2 is a rocker arm 4 whose one end is in contact with a cam face of the valve driving cam 2a. The other end of the rocker arm 4 is in contact with an upper end of a stem of the intake valve 3. The rocker arm 14 has a convexly curved back surface 5 to which a lever 6 contacts, so that the pivotal movement of the rocker arm 5 caused by the cam 2a is carried out having the curved back surface 5 controlled by the lever 6. That is, the lift of the cam 2a is transmitted to the intake valve 3 while being controlled by the lever 6. The lever 6 has one end rotatably supported by a supporting shaft 7 and has a sloped upper surface 6a against which a control cam 8 abuts. Denoted by numeral 9 is a valve spring by which the intake valve 3 is biased upward, that is, in a direction to close the intake port.
During engine operation, due to a known driving mechanism, such as hydraulic actuator or the like, the rotation of the control cam 8 is forced to change its phase in accordance with the engine condition, so that the valve timing and the valve lift of the intake valve 3 are continuously controlled. That is, when the lever 6 is depressed by a larger degree by the control cam 8, the contact of the rocker arm 4 to a base circle part of the cam 2a induces a close positioning of a free end of the lever 6 to the rocker arm 4, and thus, the valve open timing of the intake valve 3 is advanced and the valve lift of the same is increased. While, when the lever 6 is depressed by a smaller degree by the control cam 8, the contact of the rocker arm 4 to the base circle part of the cam 2a induces a remote positioning of the free end of the lever 6 to the rocker arm 4, and thus, the valve open timing of the intake valve 3 is delayed and the valve lift of the same is reduced.
However, due to its inherent construction, the above-mentioned conventional valve mechanism has to employ inevitably an arrangement wherein the cam shaft 2 and the valve driving cam 2a are placed at a middle zone of the upper deck of the cylinder head 1. Accordingly, in internal combustion engines having a plurality of intake or exhaust valves for each cylinder and thus having a plurality of valve driving cams for each cylinder, the freedom in layout of an associated ignition spark plug (not shown) relative to the valve driving cam 2a and the common cam shaft 2 on the upper deck of the cylinder head 1 is considerably limited and thus locating the ignition spark plug at a desired position in the associated combustion chamber is sometimes impossible or at least very difficult.